Experimental analysis on the change regulation of the soil resistivity considering the thermal effect around the grounding electrode

When the HVDC system operates in the monopole ground return mode in a long duration, a continuous current with high magnitude travels into the earth through the grounding electrode, leading to temperature rise to the neighbouring soil. In the extreme condition, the soil agglomerates and the grounding resistance increases consequently, threatening the operation safety of the power system. In the current design of the DC grounding electrode, the soil temperature rise is calculated based on the simulation. However the soil resistivity is treated as a constant in the existing simulation model, without consideration for the soil resistivity change due to the temperature rise and water loss of the soil. According to the present results, the maximum temperature rise of the grounding electrode may reach 80°C, thus the simulation experiment is conducted to study the soil resistivity change regulation as the result of the water loss and the temperature rise from 20°C to 80°C in this paper. From the experiment result it is found that when the temperature reaches a certain value, the dry layer appears on the soil surface. With further accretion of the temperature, the soil resistivity increases rapidly, and will go up exponentially with the increase of the dry layer thickness.